Make span_of sound

[RalfJung]

Fixes https://github.com/Gilnaa/memoffset/issues/52

While at it I also fixed some minor things:

• rename _memoffset_offset_from macro to reflect that it is unsafe to call (internal macro, so this is not semver-breaking)
• test all versions on Travis that are significant in build.rs

[Gilnaa]

Great work!